1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas. Specifically, it relates to a catalyst which can efficiently purify nitrogen oxides (NO.sub.x), included in an exhaust gas, even if the exhaust gas contains oxygen more than necessary for oxidizing carbon monoxide (CO) and hydrocarbons (HC).
2. Description of the Related Art
As a catalyst for purifying an automotive exhaust gas, there has been employed a 3-way catalyst so far which oxidizes CO and HC and simultaneously reduces NO.sub.x. For example, the 3-way catalyst has been known widely which comprises a honeycomb-shaped substrate formed of cordierite, or the like, a catalyst carrier layer formed of .gamma.-alumina, or the like, and formed on the substrate, and a noble metal catalyst ingredient selected from the group consisting of platinum (Pt), palladium (Pd) and rhodium (Rh), and loaded on the catalyst carrier layer.
The purifying performance of the conventional exhaust gas purifying catalyst depends greatly on an air-fuel ratio of a fuel which is supplied to an engine. For example, when the air-fuel ratio (i.e., A/F ratio) is large, or when the air-fuel ratio is on a fuel-lean side (i.e., when a concentration of a fuel is small in an air-fuel mixture ), the content of oxygen is large in an exhaust gas emitted from the engine. Thus, the oxidizing reactions of purifying CO and HC are active, but the reducing reactions of purifying NO.sub.x are inactive. On the other hand when the air-fuel ratio is small, or when the air-fuel ratio is on a fuel-rich side (i.e., when a concentration of a fuel is large in an air-fuel mixture), the content of oxygen is small in an exhaust gas emitted from the engine. Thus, the oxidizing reactions are inactive, but the reducing reactions are active.
When driving an automobile, especially when driving an automobile in urban areas, the automobile is accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (air-fuel ratio: 14.6) to the fuel-rich side (i.e., in an oxygen-lean atmosphere). In order to satisfy the low fuel consumption requirement under the driving conditions such as in the above-described urban areas, it is necessary to operate an automobile on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engine. However, the thus lean-burn-controlled engine emits an exhaust gas which contains oxygen in a large amount. As a result, the reducing reactions of purifying NO.sub.x are inactive. Hence, it has been desired to develop an exhaust gas purifying catalyst which can satisfactorily purify NO.sub.x included in the exhaust gas which is emitted from the lean-burn-controlled engine, and which contains oxygen in a large amount.
Under the circumstances, a novel exhaust gas purifying catalyst was proposed, for example, in Japanese Unexamined Patent Publication (KOKAI) No. 4-118,030. This exhaust gas purifying catalyst employs a catalyst carrier layer formed of mordenite which has an HC adsorbing ability. When the temperature of an exhaust gas is low, the exhaust gas purifying catalyst adsorbs HC. When the temperature of the exhaust gas increases, the exhaust gas purifying catalyst releases HC. The released HC reduce No.sub.x included in the exhaust gas. As a result, the exhaust gas purifying catalyst can exhibit an improve NO.sub.x conversion.
Another novel exhaust gas purifying catalyst was proposed, for example, in Japanese Unexamined Utility Model Publication (KOKAI No. 6-69,317. As illustrated in FIG. 11, this exhaust gas purifying catalyst comprises a substrate 1, a catalyst carrier layer 2, and a catalyst ingredient (not shown), and is subjected to a supply of supplementary HC from an upstream side of an exhaust gas flow. The supplementary HC can be propylene, or the like. When an exhaust gas is in an oxygen-rich atmosphere, the supplementary HC are forcibly supplied to the exhaust gas purifying catalyst from an upstream side of an exhaust gas flow. Consequently, HC are compulsorily supplied to and absorbed on the catalyst carrier layer 2 so that the oxidizing reactions, which the absorbed HC effect, decrease the atmosphere around the active cites of the exhaust gas purifying catalyst down to the stoichiometric point (air-fuel ratio: 14.6). The exhaust gas purifying catalyst thus purifies NO.sub.x, which are included in the exhaust gas of an oxygen-rich atmosphere.
The exhaust gas purifying catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 4-118,030 ( has the catalyst carrier layer which is formed of mordenite. The mordenite exhibits an HC absorbing ability. However, the exhaust gas purifying catalyst was found to have the following disadvantages when the exhaust gas purifying catalyst was tested under the following testing conditions:
a 10%-fuel-lean air-fuel mixture was combusted in an automobile engine; PA1 the resultant exhaust gas included NO in an amount of 250 ppm, HC in an amount of 1,000 ppmC (a value converted into a carbon content), O.sub.2 in a amount of 10% by volume, CO in an amount of 150 ppm, CO.sub.2 in an amount of 6.5% by volume, and the balance of N.sub.2 ; and PA1 the exhaust gas flowed at an SV (i.e., space velocity) of 67,000 hr.sup.-1. As illustrate in FIGS. 10 (A) and 10 (B), the exhaust gas purifying catalyst exhibited an increasing conversion against HC included in an exhaust gas. The HC conversion increased to 100% as the temperature of the exhaust gas elevated. On the contrary, the exhaust gas purifying catalyst exhibited a decreasing conversion against NO.sub. x, included in the exhaust gas, as the temperature of the exhaust gas elevated. PA1 a substrate; PA1 a catalyst carrier layer formed on the substrate; PA1 a catalyst ingredient loaded on the catalyst carrier layer; and PA1 means for supplying hydrocarbons into the catalyst carrier layer, the means disposed in the substrate.
The disadvantageous phenomena are believed to occur as follows. The released HC is effected to preferentially carry out their own oxidizing reactions by the catalyst ingredient. As a result, the reducing reactions of NO.sub.x are less likely to take place.
In this instance, even if HC are supplied from an upstream side of an exhaust gas flow as illustrated in FIG. 11, HC cannot be utilized effectively to purify NO.sub.x in an elevated temperature range. This drawback is believed to be caused by the following mechanism. In an elevated temperature range, HC have been oxidized independently around the surface of the catalyst carrier layer 2, and cannot be diffused into the inner side of the catalyst carrier layer 2.